Sustainable economic growth in the 21st century would inevitably result in a massive expansion in the use of renewable energy technologies. Solar energy is, by far, the most plentiful renewable source of energy. It is generally known that more solar energy strikes the cross-section of the earth in one hour than humanity currently uses in one year. Therefore, research efforts in the renewable energy arena are expected to focus on direct capture and utilization of solar energy.
The spatial and temporal intermittency of solar energy requires the development of large-scale capture and storage technologies. Without storage, solar energy cannot replace fossil fuels as the dominant source of energy, especially in the transportation and industrial sectors. To this end, many researchers have focused efforts on developing new means of storing large quantities of solar energy. One approach involves biological engineering of photosynthetic organisms to provide advanced biofuels. Another involves continued development of inexpensive and efficient solar photovoltaics to couple to advanced secondary batteries such as sodium-sulfur and lithium ion cells. A third approach involves so-called “artificial photosynthesis,” wherein a semiconductor or photocatalyst material is made to produce a chemical compound (fuel), such as hydrogen, which can be stored and transported as needed.
Regardless of the specific processes involved, an energy system based on sunlight input generally requires three components: the means to capture solar photons, a way to store the energy of those photons, and a method to release the stored energy for use on demand. Notably, fossil fuels also fit this descriptive model of solar energy storage and release. However, energy storage in fossil fuels proceeds over geological time scales. To date, the development of cost-competitive renewable alternatives to fossil fuels has been challenging. For all these reasons, improved methods for the efficient storage and release of solar energy would result in a significant advance for renewable energy usage.